This is probably the last constituency Gov. Dannel P. Malloy was counting on to fill in the circle next to his name on the ballot: gun owners.

But should Republican John McKinney win his party's nomination to challenge the first-term Democrat, a founding member of a key Second Amendment group said Tuesday he would have no choice but to support Malloy in 2014.

He criticized McKinney, the GOP's lead negotiator on a package of gun control measures adopted by Connecticut in response to the shootings at Sandy Hook Elementary School, for selling out.

McKinney's baptism as an official candidate for the state's highest office elicited immediate criticism from the right and left, with allies of the state Senate minority leader from Fairfield saying he is uniquely positioned to mount a serious run from the political center.

McKinney, 49, who represents Fairfield, Westport, Weston, Easton and, notably, Newtown, filed paperwork with the state Elections Enforcement Commission on Tuesday to run for governor. He also unveiled a campaign website.

McKinney is the first candidate to formally declare in what has the potential to be a bellwether race. Malloy who is expected to wait until later this year to announce his re-election bid.

Former U.S. ambassador to Ireland Tom Foley is the early favorite to represent the GOP after coming within 6,500 votes of beating Malloy in 2010.

"John is getting into the race because Gov. Malloy has done such a lousy job of running Connecticut that Republicans know there is an opportunity to defeat his progressive agenda that has landed Connecticut dead last in economic growth," Foley said. "Gov. Malloy was booed last week for a reason.

"Taxpayers, workers and people trying to take care of their families have lost confidence in Dan Malloy and his misguided policies and leadership. It's time for a change. It is no surprise that people with strong leadership records are jumping into that void."

Danbury Mayor Mark Boughton, currently unopposed for a seventh term, is also considering a run for governor.

"John's a good man and certainly will lend a voice to the debate of the issues that confront our state," Boughton said. "My decision is going to be based on what is the best thing for the state, first, and, obviously, for me and my family. I never let anybody else dictate what decisions I make based on whether they're running or not running."

Former U.S. Rep. Christopher Shays questioned the logic of the gun lobby and called it short-sighted, saying McKinney is the most capable of restoring the economic promise of Connecticut.

"It makes no sense. It makes no sense," said Shays, who succeeded McKinney's father, Stewart B. McKinney, in Congress upon his death from AIDS in 1987.

Democrats quickly sought to typecast McKinney as out-of-step with the middle class and opportunistic when it comes to seeking taxpayer dollars for his upcoming campaign from the citizens election fund program, which McKinney recently told Hearst Connecticut Newspapers he plans to do.

"I do think that it's interesting that he was so opposed to it and yet now he is willing to use it," said Nancy DiNardo, the state Democratic chairwoman. "Is it a perfect system? No. But I think it's a very good system. It gives candidates the ability to talk to the voters about issues rather than always worrying about raising money."

McKinney must raise $250,000 in maximum donations of $100 to qualify for $1.25 million for a primary and $6 million for the 2014 general election under the program.

"Obviously, I don't like the fact that tax dollars are used to fund our campaigns," McKinney said. "I do not have the wherewithal to be a self-funder."

The top Republican in the Democratic-controlled Legislature, McKinney stood by his support of universal background checks on the purchase of firearms and ammunition, capping the size of gun magazines and expanding the definition of assault weapons to include the type of semi-automatic rifle used to kill 20 children and six educators last December in Newtown.

"I understand that there are people who disagree with that vote," McKinney said.

For the GOP, which hasn't won a statewide election since 2006, McKinney's record on gun control could turn the party's nominating contest into a referendum on Second Amendment issues.

However, state GOP Chairman Jerry Labriola Jr. said, "This election will be decided on the economy." He categorized McKinney as a "top-tier" candidate.